In various applications, it is necessary to acquire precise information regarding the position of an object. The object of interest may be fixed in position or may be a movable one. By way of example, positioning systems and measuring systems that are used in the integrated circuit fabrication industry must have a high level of accuracy. Prior to wafer dicing, an array of identical integrated circuits is formed on a semiconductor wafer by stepping the wafer relative to a system or system component, such as an image-bearing reticle. Often, both the reticle and the wafer are connected to stages which are movable. As used herein, a “wafer stage” includes both an apparatus for supporting the wafer and/or the apparatus for supporting the reticle.
A typical wafer stage is movable in perpendicular X and Y directions. The wafer stage can therefore be stepped after each exposure of the wafer. For example, in the use of a reticle, a photoresist layer may be repetitively exposed onto a wafer by projecting an image of the reticle through a projection lens to one area on the wafer, stepping the wafer stage, and repeating the exposure. The wafer is scanned using the X and Y movements of the wafer stage until each integrated circuit region is properly exposed. In addition to the movements in the X and Y directions, Z axis movement is enabled. In wafer lithography, the Z axis may also be considered the exposure optical axis or the “focus” axis. The required range of motion in the Z direction is significantly less than the necessary ranges in the X and Y directions.
Acquiring position information regarding movement of a wafer stage in the Z direction is somewhat more problematic than acquiring such information for X and Y movements. An approach to providing Z axis measurements is to use an encoder that employs interferometric techniques. One concern with this approach is that interferometer components must be relatively large in order to capture the diffracted orders as the stage translates through its full range, since the required diffraction angle must be relatively great in order to achieve the target accuracy. As an alternative, a standard Michelson interferometer may be used to monitor Z axis motions. However, if the measurement is performed from the projection lens side of the wafer stage, the percentage of stage real estate that is available to the wafer or reticle must be smaller (for a given size stage), since the laser light from the interferometer should not impinge the wafer or reticle. On the other hand, if the measurement is performed from the side of the stage opposite to the projection lens, the measurement system must use an intermediate reference, such as the stone below the stage. Among other potential disadvantages, this requires a separate measurement of the stone relative to the projection lens.
FIG. 1 illustrates another approach to acquiring position information of a wafer stage 10 along a Z axis. This approach is described in detail in U.S. Pat. No. 6,208,407 to Loopstra. A wafer 12 is shown as being supported on the stage for exposure by projection optics or exposure tool 14. The advantage of this approach is that although the interferometer 16 is positioned at the side of the stage 10, accurate Z axis measurements may be obtained. This is enabled by properly positioning mirrors which establish a Z measuring axis 18 that is parallel to the Z axis 20 of the exposure system. A first mirror 22 is arranged at a forty-five degree angle to movement of the stage 10 along the X or Y direction. A measuring beam 24 from the interferometer impinges the forty-five degree mirror to establish the Z measuring axis 18. A horizontal mirror 26 is attached to structure 28 of the exposure system, so that the beam is redirected to the first mirror 22, which reflects the returned beam to the interferometer 16. In addition to the measuring beam 24, the interferometer projects a test beam 30 for reflection from a vertical surface 31 of the stage 10.
As can be seen in FIG. 1, movement of the wafer stage 10 along the Z axis 20 will result in a change in the length of the beam path segment from the forty-five degree mirror 22 to the horizontal mirror 26. Thus, while the interferometer 16 is located at the side of the stage, the measuring beam 24 has a path segment that varies in length in unity with Z axis displacements of the stage. In fact, the reflection from the horizontal mirror 26 to the forty-five degree mirror provides a second beam path segment that varies in unity with Z axis movement of the stage. On the other hand, the length of each beam path segment for the test beam 30 is fixed, unless the stage 10 is moved in the X direction.
While the approach described with reference to FIG. 1 operates well for its intended purposes, there are cost concerns, since the horizontal mirror 26 is a relatively large reflective component that requires a high degree of planarity. Moreover, as the linewidths of the features of integrated circuits decrease, the size of the projection lens of the projection optics 14 increases. In FIG. 1, this would result in an increase of the diameter of the projection optics. As a consequence, the requirement of a horizontal mirror 26 to accommodate the entire range of motion of the stage imposes a potential difficulty with respect to achieving further reductions of linewidths.
For systems in which the increase in size of a projection lens is not an issue, there may be other reasons for avoiding the use of a horizontal mirror of a similar type and orientation of FIG. 1.